Easy to build projects for everyone - WorldRadioHistory.Com · 74 SERIES TTL IC's TESTED TRANSISTOR...

Easy to build projects for everyoneSEPT. 77

35p

11 11 11

-it

JUBILEE WILE74 SERIES TTL IC's TESTED TRANSISTOR PA KS

Type Quantity Type Quantity Type Quantity All devices brand new, tested and coded1 100 1 100 I 100tp Et, fpfp ip Ep

7400 0.09 0.08 7448 0'70 0.68 74122 0.45 0.427401 0-11 0.10 7450 0.12 0.10 74123 0.65 0.627402 0.11 0.10 7451 0-12 0.10 74141 0.68 0.657403 0.11 0.10 7453 0.12 0.10 74145 0.75 0-727404 0-11 0-10 7454 0.12 0.10 74150 1.10 1.057405 0-11 0-10 7460 0.12 0.10 74151 0.65 0.607406 0-28 0.25 7470 0'24 0.23 74153 0.70 0.687407 0.28 0.25 7472 0.20 0.19 74154 1-20 1'107408 0.12 0.11 7473 0.26 0-22 74155 0-70 0.687409 0-12 0-11 7474 0.24 0.23 74156 0.70 0.687410 0.09 0-08 7475 0 44 0.40 74157 0.70 0.687411 0.22 0.20 7476 0.26 0.25 74160 0.95 0.857412 0.22 0.20 7480 0.45 0.42 74161 0.95 0-857413 0.26 0.25 7481 0 90 0.88 74162 0.95 0-857416 0.28 0-25 7482 0.75 0.73 74163 0.95 0-857417 0.26 0-25 7483 0 88 0.82 74164 1-20 1.107420 0-11 0-10 7484 0.85 0-80 74165 1.20 1.107422 0.19 0.18 7485 1.10 1.00 74166 1-20 1.107423 0.21 0.20 7486 0.28 026 74174 1-10 1.007425 0-25 0.23 7489 2.70 2-50 74175 0.85 0-827426 0-25 0.23 7490 0.38 0.32 74176 1.10 1.007427 0-25 0.23 7491 0.65 0-62 74177 110 1.007428 0.36 0'34 7492 0-43 0-35 74180 1-10 1.007430 0.12 0-10 7493 0-38 0-35 74181 1.90 1-807432 0.20 0-19 7494 0.70 0.68 74182 0.30 0.787433 0.38 0-36 7495 0.60 0.58 74184 .50 1.407437 0-26 0.25 7496 0.70 0-68 74190 '40 1.307438 0 26 0.25 74 00 0.95 0.90 74191 .41 1.307440 0.12 0-10 74 04 0.40 0.35 74192 10 1007441 0.60 0.57 74 05 0.30 0.25 74193 05 1.007442 0.80 0-70 74 07 0.30 0.25 74194 An 1.00 )49 2x I amp I x4001 x6007443 0.95 0.90 74 10 0.48 0.45 74195 0 80 0-75 J50 5anip SCR I x 50 2 x 400 1 x6007444 0.95 0-90 74 II 0.75 0'72 74196 0-90 0.857445 0.80 0-75 74 18 0-85 0-82 74197 0-90 0.857446 0.80 0.75 74 19 1-30 1.20 74198 I90 1.807447 0-70 0.68 74 21 0-28 0.26 74199 I80 I70Devices may be mixed to qualify for quantity price. Data is avail-able for the above series of IC's in booklet form Price 35p

CD4000CD4001CD4002CD4006CD4007CD4008CD4009CD4010CD4011CD4012CD4013CD4015CD4016CD4017CD40113CD4019CD4020C04021

LO 18LO 18CO 18L0 80LO 18LO 80L0 50L0 5040 18CO 18LO 42LO 80LO 42CO 80LO 8560-45L0 95LO -85

CMOS ICsCD4022 LO 80CD4023 LO 18CD4024 LO 64CD4025 LO 18CD4026 11-85CD4027 10.48CD4028 CO.80C04029 LO 95CD4030 L0'46C04031 LI 80CD4035 LI 40CD4037 LO 78CD4040 LO '78CD4041 E0.68CD4042 E0 '68CD4043 60.78CD4044 LO 78CD4045 L1.15

CD4046 L0.95CD4047 LO 75CD4049 10.46CD4050 L0.46CD4054 10.95CD4055 11.60CD4056 fl 15CD4069 10.32CD4070 LO 32CD407 I LO 20CD4072 f0.20CD4081 L0.20CD4082 LO 20CD4510 LI .10CD45 I I 11-25CD4516 L1.10CD4518 LI .10 J57 54 first quality miniature electrolytics from -47µF-1000pS El -20*CD4520 CI -10

J1 3 each AC128/ACI7612 10 x 13C107J3 10 x 8C108J4 10 x BC10935 3 each BC148/149-2 BC I 47*J6 3 each 8C169/17 I / 1 72.J7 2 each 8C177/8/9J8 2 each BC I 82/3/4.19 2 each BC212/213/214.110 2 x BC327 3 x BC328.JI 12 x BC337 3 x BC338)12 2 each BF115-8F167-BF173113 2 each 0F1941516.)14 2 x BF258115 2 x BFX29 3 x BFX84

PRICE 60p PER PACK

TESTED DIODE PA KSJ30 12 x 0A81131 12 x 0A91J32 12 x 0A200)33 12 x 0A202)34 15 x1N4148

116 2 each BFY50/51/52J17 6 x 0071118 4 each ZTX108/9 2 x ZTXI07119 2 each ZTX301/2 4 x ZTX300120 2 each ZTX500/1/2.J2I 4 each 2N706/2N708J22 I each 2N2218/19/21/22.I23 2 each 2N904/05J24 3 each 2N2907 2 x 2N2906125 7 x 2N2926 GJ26 4 x 2N3053J27 2 x 2N3055J28 3 each 2N3702/03/04.129 3 each 2N3904/06.

J35 20 x IN4001)36 15 x IN4002J37 10 x IN4004138 8 x IN4007

PRICE 60p PER PACK

)39 5 x 1N5400J40 5 x IN5401141 4 x IN5404342 3 x IN5408

OPTOELECTRONICS)43 4 x DL707 + DataJ44 5 x -125 Red LEDJ45 5 x .2 Red LED146 5 x -2 and -125 LED's mixed colours147 3 x ORPI2 equivalent148 5 x OCP71

THYRISTOR PACKS

L2.8010.50L0.50L0.50LI 20LI 00

LI -00CI 50

UNIJI UNCTION F.E.T. PAKS151 6 x TIS43/UT46 LI .00J52 4 x 2N38I9 £0.60

P.C.B. PRODUCTS

ORDERINGPLEASE WORD YOUR ORDERS EXACTLY AS PRINTEDNOT FORGETTING TO INCLUDE OUR "J" NUMBERS

V.A.T.ADD I2i% TO PRICES MARKED " ADD 8% TO ALLOTHERS

POSTAGE AND PACKINGADD 2Sp FOR POSTAGE AND PACKING UNLESSOTHERWISE SHOWN. ADD EXTRA FOR AIRMAILMINIMUM ORDER £100

J53 2x Etch resistant pens LI '00)54 2 x etchant paks L1.00

D.I.Y. PRINTED CIRCUIT KITCONTAINS 6 Pieces copper laminate, box of enchant powder andmeasure, tweezers, marker pen, high quality pump drill, Stanley knife& blades, 6 inch metal rule. FULL Easy to follow instructions.

J59 ONLY I5.50

RESISTOR PAKJ55 240 first quality g/I-VV resistors -mixed from 100ohin-820k LI .60156 160 first quality -}-W resistors -mixed from 100ohm-820k El .60

ELECTROLYTIC PAK

CERAMIC PAKJ58 93 first quality ceramic capacitors -mixed from 22pF-047µF LI -60*

I.C. SOCKET PAKS360 I 1 x 8 pin D.I.L. SocketsJ6I 10 x 14 pin D.I.L. SocketsJ62 9 x 16 pin D.I.L. Sockets

ZENER PAKSJ63 20 mixed value 400MW zener diodes 3-10V164

CI 0000

11.00

II -0020 mixed value 400MW zener diodes 11-33V 11-00

Dept. E.E.9, P.O. Box 6, Ware, HertsCOMPONENTS SHOP.18, BALDOCK STREET, WARE, HERTS.

11

VOL. 7 NO. 1

CONSTRUCTIONAL PROJECTSPROBOPHONE Simple to use electronic organ by D. C. JenkinDYNAMO BACK-UP Useful aid to the cyclist by R. EversonFLASHER BLEEPER Attracts the attention of the forgetful driverADD-ON CAPACITANCE UNIT Turn your multimeter into a capacitance meter

SEPTEMBER 1977

8

14

by T. R. de Vaux-Balbirnie, B.Sc. 24

by R. A. Pen fold 32

GENERAL FEATURESEDITORIALSHOP TALK New products and components for constructional projects by Brian TerrellTHE OSCILLOSCOPE How it works, what it does by John SmithSQUARE ONE Beginners! Start Here!BRIGHT IDEAS Reader's hints and tipsBOOK REVIEWS A selection of recent releasesPHYSICS IS FUN Shocking coils by Derrick DainesREADERS' LETTERS Your news and viewsRADIO ASTRONOMY AT CAMBRIDGE Looking at the stars with radioWHAT DO YOU KNOW? Test your knowledge-PowerYOUR CAREER IN ELECTRONICS Selling components by Peter VerwigJACK PLUG AND FAMILY CartoonFOR YOUR ENTERTAINMENT Video cassettes by Adrian HopePROFESSOR ERNEST EVERSURE The Extraordinary Experiments of.DOWN TO EARTH Mains hum and earth by George HyltonPLEASE TAKE NOTE Touch Switch, Fish Attractor, Fuzztone, Soil Moisture Monitor

Back Number Service and BindersBack issues of EVERYDAY ELECTRONICS (June 1977 onwards) are avail-

able worldwide at a cost of 60p per copy inclusive of postage and packing.Orders and remittance should be sent to: Post Sales Department, IPCMagazines Ltd., Lavington House, 25 Lavington Street, London SE1 OPF.

Binders for Volumes 1 to 6 (state which) are available from the above addressfor £2.10 inclusive of postage and packing.

© IPC Magazines Limited 1977. Copyright in all drawings, photographs and articlespublished in EVERYDAY ELECTRONICS is fully protected, and reproductions orimitations in whole or in part are expressly forbidden.

All seasonable precautions are taken to ensure that the advice and data given toreaders are reliable. We cannot however guarantee it, and we cannot accept legal re-sponsibility for it. Prices quoted are those current as we go to press.

SQUARESfrxeatiPelftfor BEGINNERS

ONE

6

13

18

19

20

22

23

27

by S. McClelland 28

35

36

39

40

by Anthony J. Bassett 42

44

44

Everyday Electronics, September 1977 7

INTRODUCTION

JABy D. JENKIN

Small hand held electronic organs have become very popular inrecent years. The following design illustrates how such an instru-ment can be constructed quite simply using just one integratedcircuit, the 556 dual timer.

HOW IT WORKS...

J

LOW FREQUENCYOSCILLATOR

VIBRATO ( DEPTH)

RESISTOR VALUESDETERMINE OSCILLATORFREQUENCY

OSCILLATOR

For the main oscillator of the Probophone to function, the circuit needs tobe completed by the addition of a resistor. This is accomplished by meansof a probe(from the circuitry) selecting via a keyboard arrangement, one of arange of resistances connected to the keyboard notes and wired back tothe circuit. By touching the probe on one of the "note -pads", a tone isheard in the loudspeaker. The resistances are chosen so that the twentytones produced are in accordance with a 14- octave musical scale.

A second oscillator running at a much lower frequency, about 10Hz, isfed to a point in the master oscillator that causes a change in frequency,to-and-fro about that selected by the probe to produce a warbling orvibrato effect.

STARTHERE FOR

CONSTRUCTION

Excluding the 20 presets, most ofthe circuitry is mounted on a pieceof stripboard having 17 strips by25 holes. The layout of this boardis shown in Fig. 2. which also showsthe connections required to VR1,the loudspeaker and the keyboard,etc. Alternatively, if the construc-tor wishes, this part of the circuitmay be incorporated on a printedcircuit board, Fig. 1.

Refering to Fig. 3. this shows thecopper pattern required for thekeyboard and presets. To makelife easier, use is made of stickylabels to define the area for the"keys". Using this particularmethod the copper pattern can bedrawn more accurately.

Begin by cleaning the copperboard thoroughly using either steelwool or a scouring powder. Ensure

8 Everyday Electronics, September 1977

KEYBOARD PROBE

1 +11°--11111WIIIIIIIL111111111

TO KEYBOARD

G)G1030PC:10J2x

Fig. 1. if the circuit is to be built on a PCB then this is the required copper pattern.Also shown is the component layout on the plain side of the board.

Two views of the PCB versionOn the left a topside viewshowing the components, andon the right the undersideshowing the copper pattern.

DIRECTION OF z-COPPER STRIPS

1 2 3 4 5 6 7 8 9 10 12 14

C3

16

000 00000 0R2

IC1

4 0-(11

4 O H4

4

4

4

410

00O R3

0

O 0 0 0 0 4111H0011151O 0 C1

0 0 0 0 0

18 2400 0000000000

RI

20 22

0 0 0 0 0 0

0= BREAKS IN COPPER STRIPS 0= VEROPINS

TO LS1

Photograph of the corn- Fig. 2. Stripboard layout also showing the breaks required on the underside. Thispleted stripboard layout. method of construction is preferred if the constructor is unsure about making a PCB.


CIRCUITDESCRIPTION

OPERATION OF THE556 TIMER

Referring to Fig. 5, this shows ablock diagram of one of the twotimers contained in the i.c. Theexternal components are connec-ted in such a way to produce acontinuously running multivibra-ter. The two resistors RA and RB,and the capacitor C set the fre-quency of oscillation.

The capacitor C charges up ata rate dependent on the values ofRA and RB. As the capacitorcharges the voltage across it risestowards the threshold voltage inthe timer. The comparator, 1, sen-ses this voltage and causes the flipflop to change state, in doing so itturns transistor TR1 on.

This has the effect of putting ashort circuit between the junctionof the two resistors and earth. Asthis happens the capacitor beginsto discharge through resistorRB. As this happens the capacitorfalls the second comparator 2 de-tects this voltage drop, which is infact a value similar to the thres-hold voltage, and causes the flipflop to revert back to its originalstate. Transistor TR1 is then turnedoff, which allows the capacitor tocharge up once again.

The output appears at pin 5 andis essentially a square wave. This

Rq

DISCHARGE 1

R8

THRESHOLD 2

TRIGGER 6

Vcc 4

TR1

14 RESET

COMP 2

5 OUTPUT

./4.1"3 CONTROL VOLTAGE

THRESHOLDVOLTAGEREFERENCE

TRIGGERVOLTAGEREFERENCE

0

Fig. 5. A block diagram of one of the timers in the 556 timer I.C.

is depicted in Fig. 6. where the"on" time of tl and the "off" timeof t2 is given by the following:

t1=0.7X(RA+RB)XCt2=0.7XC

The frequency is then given by:f =1/T cycles. I

t1-0.1kw- 7-31.

Fig. 6. Wave form appearing at pin 5 of thecircuit shown in Fig. 5.

PROBE]

ETC.

20 x 20k12 TRIMMERS VR2-21

151

R1

5612

CI100pF

f1i2

ikn

5

T2022pF

7

14

3

10 14

13

MIIIIMOMB C6

0.33pF

C5

VR1 0.2.2pF;Oka

VIBRATO

TB19V

Fig. 7. Complete circuit diagram of the Probophone.

CIRCUIT DESCRIPTIONThe dual timer is connected as

two separate astable multivibra-tors; a tone oscillator, the fre-quency of which depends on thevalues of the presets, and a vibra-to oscillator running at about 10Hz.

The threshold, pin 2 of the toneoscillator is connected to the 20presets mounted on the keyboard.These presets are in fact thevarious values of RB alreadydescribed, which go to make upthe musical scale. The vibratooscillator comprising the secondhalf of ICI together with R4, R5and C5 produces a 10Hz square -wave at the output, pin 9.

The ideal vibrato should pre-ferably be a sinewave, in order toapproximate this shape, a filterconsisting of R3 and C3 is used tofilter the high frequencies causedby the sharp edges of the square -wave. The vibrato level, controlVR1, passes the resulting wave-form to the control voltage, pin 3of the tone oscillator, thus causingthe familiar rythmic wobble to thesound.

Resistor R1 limits the output ofthe i.e. to well within its capabili-ties and also reduces battery con-sumption. Capacitor CI blocksdirect current to the loudspeaker.The capacitor across the supplyprevents instability occurring whenthe internal resistance of the bat-tery increases with age. If it is notfitted this will cause slight ploppingnoises to be heard from the loud-speaker.


PLACI30P110atFig. 4. Dimensions of thecase made from 4mm ply-wood. Depending on thecomponents used, the sizesmay have to be altered.Below; layout of the presetson the copper side of thePCB. The resistors are nor-mally replaced with wirelinks. If it is desired to usefixed values for the tuning,then these are used and thepresets replaced with wirelinks. The resistors selectedare then soldered in thepositions indicated by Ra,Rb, etc.

41

3

73

0= 76x25 x60 = 152 x 13 x 13

ALL DIMENSIONS IN mm

In this photograph, fixedvalue resistors have beenused in place of thepresets.

Fig. 3. Copper pattern required for the keyboard. This is drawn full size and may be used as a template.


COMPONENTS--i;Resistors

R1 1k0R2 560R3 330k0R4 47kQR5 330k0Ra to Rt (see text) All resistors carbon +W + 10%

PotentiometersVR1 10kS2 lin. carbonVR2 to 21 22k0 horizontal sub -miniature presets (20 off)

CapacitorsC1 100pF 16V elect.C2, C3 0.22uF polyester (2 off)C4 0.01tiF polyesterC5 0.22u F polyesterC6 0.33,uF polyester

SemiconductorIC1 NE556 dual timer i.c.

MiscellaneousB1 9V PP3 batteryS1 s.p.s.t. toggle switchLS1 3 to 5 ohm speaker approx 60mm diameterStripboard 0.1 inch matrix 17 holes x 25 holes; battery connector; i.c.socket; small knob for VR1 ; veropins; modified ball point pen for probe;printed circuit board, size as required (see text); materials for case (seetext); connecting wire; solder; etc.

See

Talicppage 13

that the board is not touched oncethis has been done. Apply thelabels with firm pressure toexclude any air bubbles, and fill inthe connections between the keysand the remainder of the circuitwith an etch resist pen.

The circuit board can then beetched in a solution of ferricchloride until all the copper hasbeen etched away. Take care whenhandling ferric chloride, it is highlycorrosive and dangerous to theskin. Once all the copper has beenremoved rinse the board in water,applying some scouring powder toremove all traces of the ferricchloride. The mounting holes forthe presets can then be drilledusing a lmm diameter drill.

In time the copper becomes dull

mooANc&

,oI.- . myth,. ESTIMATED COSTOF COMPONENTS

£5

and tarnished with use, and un-less the printed circuit board isplated it is necessary to give thekeys an occasional rub with metalpolish to prevent poor contact withthe probe.

CASE DETAILSThe prototype Probophone was

constructed in a simple plywoodcase. There is of course no reasonwhy a ready made case of similardimensions cannot be used.

The general layout of com-ponents in the case is shown inthe various photographs. The sizesgiven in Fig. 4 are of course onlycorrect if t h e recommended

printed board and loudspeaker areused. If alternative components areused then the dimensions need tobe altered.

The vibrato control is mountedon the rear panel, although itwould be an advantage when con-structing the case, provision ismade for it to be mounted on thefront panel. Once the case has beenconstructed it can be painted in acolour of the constructor's choice.The lid needs to be removable togain access to the tuning presetsand to replace the battery when re-quired.

TUNINGOnce construction has been com-

pleted and a check has been madefor any wiring errors, the batterycan be connected. Touching theprobe to one of the "keys" a noteshould be heard. Set the vibratocontrol to zero and adjust theappropriate preset for the correctpitch, if possible comparing thenote with a piano, pitch pipes orother musical source.

An alternative cheaper methodis to select fixed resistors to makeup the required values. Each noteis initially tuned by inserting a15 kilohm potentiometer in placeof the resistor. The potentiometeris then carefully adjusted and thevalue measured on an ohm meter.This value is then made up fromdifferent combinations of fixedresistors and then soldered in placeon the printed circuit board. If thisis done carefully the combinationshould produce the same note astuned by the potentiometer.

Once all the tuning has beencarried out and the constructor issatisfied that the range of notesproduced are correct the Probo-phone is then ready to be played! I=1

General view of the completed Probophone, showing the layout used.


:_',TRONIC SUPPLIES

By Brian TerrellNew products and component

buying for constructional projects.

Unfortunately the introduction subjectto last months Shop Talk, dealing withgetting together a stock of componentsfor project building suffered from lack ofspace. This is an important topic that hasbeen noted in my diary for future treat-ment.

New Soldering IronWe received news and a model (return-

able unfortunately) of a soldering ironrecently added to the Rawplug range oftools. This mains powered 25 watt ironconforms to BS 3456 and carries the BEAB(British Electrical Appliances Board)mark of approval.

It has been designed for the radio andt.v. engineer and the d.i.y. enthusiast. It issuitable for construction of E.E. projects.

Features include a slide -on iron platedconical bit which encloses the ceramicheating element housed in a stainlesssteel shaft. Insulation is quoted at 1500volts a.c.

The fully insulated handle has beendesigned to prevent rolling if the iron islaid down on the workbench. A hook isfitted to the handle for securing the ironwhen not in use. Ample three -core mainslead is attached, 1-8 metres long.

The iron is available from most d.i.y.stores and is priced at £3.80 excludingVAT.

New Marshalls CatalogueA new 32 -page catalogue is now

available from Marshall's priced at 25pto callers or 35p post paid. The cataloguetakes on a new look having much largerpages (290 x 210mm) and has been setout to make component buying easy. Inour opinion a vast improvement on theirprevious catalogue!

Due to the postal dispute in Crickle-wood, customers of Marshall's (compon-ents and catalogues) are advised todirect their mail orders to the Bristol orGlasgow branches. Addresses are to befound in the advertisement pages.

Tidy Tubs For Tools and ThingsA useful set of small cylindrical storage

compartments-six in fact-of varyinglengths and diameters bonded togetherfrom Tidy Tubs now being marketed byPlatignum, Electronics constructors mayfind these useful for holding and keepingtogether essential tools on the workbenchsuch as pliers, screwdrivers, files, drills,etc.

You won't find these for sale byelectronic component suppliers, but theyshould be available from all good depart-ment stores and stationers at a cost of£1.50. There are six colours to choosefrom.

Constructors are advised to keep theirTidy Tubs in a safe secure place as othermembers of the family will find other usesfor them in the kitchen, bedroom, bath-room or office.

Electrically Conductive PaintA product that should interest the

amateur electronic constructor hasrecently been made available to theconsumer market by Industrial ScienceLimited through several outlets (seebelow). It is electrically conductive paint,named Elecolit 340.

It can be applied by brushing, dipping,silk screen or roller and forms a toughfilm with good adhesive to ceramics,glass, rubber, plastic and most plasticfilms.

Typical applications involve r.f.shielding, printed circuit repair, in proro-type circuit production and has beenused to repair a broken track in a carrear window heater.

A colleague suggested an applicationfor car windscreen wiper control systemwhere the paint was placed on the outsideof the front windscreen to be swept bythe wipers, in the pattern of interlockingfingers to form a sensor.

The Rawplug 25W soldering iron.

Photograph of a Tidy Tub being used tohold various tools.

The control circuitry would be designedto switch the wipers on when moisturebridged the sensor and off when thewipers swept the screen dry (or below athreshold moisture level). Any offers?

We received a 3g bottle of the paint andbrushed it onto a break made along acopper track on stripboard. Two thincoats were applied at half an hourinterval and left to dry overnight. Whenmeasured the next morning with ourAvo meter, the resistance reading wasjust above zero on the divide -by -100 rangeindicating a virtual short circuit.

The paint is not cheap-understandablewhen you realise that it contains puresilver-and costs £270 for a 3 grambottle which includes VAT, postage andpacking from: Magenta Electronics Ltd.,61 Newton Leys, Burton -on -Trent, Staffs.,Xerosa Radio, 305 St. Paul's Road,Highbury Corner, London N1, and Zar-tonics, 115 Lion Lane, Haslemere, Surrey.

Constructional Projects This MonthAll the components required for the

four constructional projects featuredthis month should be readily availablefrom component suppliers advertisingin this issue.

Care should be taken when buyingcomponents for the Probophone projectespecially with regards the sub -miniaturehorizontal presets and the C280 typecapacitors. The printed circuit boardhas been designed to accommodatethese components and other sizes willnot be usable with our board design.

In the Dynamo Back -Up unit, theweatherproof case can be a standardrigid case treated with a silicon spraywhen fully assembled. This is sometimesused to make watertight the distributorhousing in a car, and should be availablefrom many garages and car accessoryshops. Don't forget to plug the grommetgap with a piece of plasticine or similar.A hose -clip or Terry clip to secure thecase to the cycle frame should be availablefrom any ironmongery shop or cycleaccessory store.


CIRCUIT DESCRIPTION

Consider first when the bicycleis in motion. The a.c. current fromthe dynamo is full -wave rectifiedby the diode bridge arrangement(DI to D4) to produce pulsed d.c.which illuminates the lamps. Pro-viding the voltage at D5 cathodeis greater than the battery voltage

voltage at junction D5/D6 fallsbelow the threshold level, thelamps will be powered by the bat-tery as D6 becomes forward biasedand passes current. Thus a mini-mum amount of light from thelamps will be maintained at alltimes.

plus the forward voltage drop ofD6 (about 0.5 volt) then D6 willbe reverse biased and the batterywill effectively be switched out ofthe circuit and so no current flowsfrom the battery.

However, when the bicycle is atrest, or travelling so slow that the

be located there with the battery.However, if a separate case is used,a suitable position must be foundfor locating it on the bicycle. If thelatter is equipped with a saddle-bag, this could be the easiestmethod, otherwise two suitablysized hose -pipe clips can be utilised.The hose -clips can be riveted orbolted to the case and the cliptightened around a circularmember of the cicycle frame. 1=1

COMPONENTS

D1 to D6 1N4001 or similar 1 Asilicon types (6 off)B1 See textS1 on/off toggleStripboard: 0.1 inch matrixsize 12 strips x 11 holes; 4 -way 2 amp terminal block;weatherproof case; rubbergrommet; battery connectorsto suit B1 selected; 4BA fix-ings-nuts, bolts and shake -proof washers (4 off); 4BAspacing pillars (2 off); hose -clips (2 off); connecting wire.

A switch has been incorporatedin the prototype to isolate thebattery from the circuit when thelights are not required. However,this may not be required as somedynamo systems incorporate aswitch in the front lamp housingand this can be wired as Si.

Fig. 2. Positioning and wiring up details tothe board and other components within thecase.

HOW IT WORKS...

GODANc ESTIMATED COSTOF COMPONENTS

£2

DYNAMO FRONT AND(.78". REAR LAMPS

A

The unit is to be placed between the dynamo and the cycle lamps. Whenthe cycle is travelling and the dynamo is being operated by the wheel, ana.c. voltage is developed which is normally applied direct to the lamps. Inthis unit, the resulting a.c. voltage is rectified first to produce d.c. and thisis then applied to the lamps via an electronic switch. With the dynamo inoperation, the battery is out of circuit (disconnected). Now if the dynamovoltage reduces below a threshold level, as a result of the cycle travellingvery slowly or stopping, the electronic switch allows the battery to powerthe lamps. Increasing the dynamo voltage above the threshold causes thebattery to return to the switched off mode.


51 mil

el It-3'1111101111111110111.11111.111111111111,

ARTICLESappearing in EVERYDAY

ELECTRONICS frequently showdiagrams of oscilloscope wave-forms. Anyone taking a seriousinterest in electronics will, sooneror later, find that an oscilloscope isneeded. Indeed, the professionalengineer can hardly manage with-out one.

It is very difficult to make useof an oscilloscope without somerudimentary knowledge of howthey work. This article sets out toexplain how oscilloscopes work,for readers who may have theopportunity to use one.

When the principles of oscillo-scopes are understood and one hasactually made use of the instru-ment, articles containing referenceto waveforms become mucheasier to understand.

CRT CONSTRUCTIONAn oscilloscope is constructed

around the cathode ray tube (CRT),which we will recall is a device forprojecting a beam of electronsonto a light emitting phosphor. Anoscilloscope CRT uses electrostaticdeflection as opposed to the mag-netic deflection employed in theCRT of television receivers.

Electrostatic deflection meansquite simply that there are twoplates mounted either side of theCRT between which the electronbeam passes. If a voltage is appliedto these plates an electrostaticfield exists between them. This

INTRODUCING the

OSCILLOSCOPE

PHOSPHOR

DEFLECTIONPLATES

ELECTRONBEAM

By JOHN SMITH

GRID CATHODE

ANODE

HEATER

Fig. 1. Electrode arrangement in a cathode ray tube.

field is capable of deflecting theelectron beam; Fig. 1 shows theelectrode arrangement.

In this tube we have a heaterto heat the cathode which gives offelectrons. The electrons passthrough a grid and are acceleratedby the anode, passing between thedeflection plates before they hitthe screen phosphor. In terms ofthe spot of light appearing on theCRT screen, the grid voltage con-trols brilliance, the anode voltagecontrols focus and the deflectionplates control position. As is shownin Fig. 1 the positign of the spotcan only be moved up and downin a straight line, therefore theCRT is fitted with another twoplates to move the spot at rightangles to the first pair. The plateswhich cause the spot to movevertically are called Y -deflectionplates, whilst the plates movingthe spot horizontally are called theX -deflection plates.

Thus we have a device where aspot of light may be controlled inany vertical position (Y -deflection),any horizontal position (X -deflec-tion), its brilliance (grid voltage)and its focus (anode voltage) arealso controlled. Most oscilloscopetubes employ more electrodes thanthis, but from a user's point ofview the only other controllablefacility they provide is the round-ness of the spot (astigmatism). Nomatter how complex and expensivean oscilloscope, they all have thesebasic tube controls.

TIMEBASEA second feature incorporated

into every oscilloscope is the timebase. This is an internal oscillatorwhich deflects the spot across thehorizontal axis. The time basecauses the CRT spot to start on theleft of the screen and travel at aconstant velocity across the screen


to the right hand side. Thus ahorizontal line is drawn which canbe evenly divided into centimetredivisions representing time.

For example, if a time base of1 millisecond caused the CRT spotto traverse 10cm across the screenthen each centimetre would repre-sent 100 microseconds of time;Fig. 2 shows how this would appearon the oscilloscope screen.

Fig. 2. Typical time scale used on oscillo-scopes.

The purpose of this time base isto show any electrical events occur-ring between the start and thefinish of the scan. The spot issuppressed (blanked) when it fliesback from the finish to the startingpoint. Oscilloscopes are providedwith a wide range of time bases toexamine slow and fast changingsignals.

At this juncture it is necessaryto examine how the time basestarts at t=0 and what is t=0?This starting point is determinedby the user of the oscilloscopewith the aid of another feature,always provided, called sync (syn-chronisation) or trigger.

The user must synchronise ortrigger the oscilloscope at the startof the signal he wishes to examine.If one is trying to make a circuitwork and it's not known if thereare signals present or not, one canbe faced with a completely blankscreen. For this reason mostoscilloscopes provide a "free run"position on the trigger circuits.

Finally, we come to the part ofthe oscilloscope which is used toexamine the circuit waveforms, theY -amplifier. The Y -amplifier is anamplifier/attenuator which trans-fers the signals to be examined tothe Y -deflection plates. If appliedwithout a time base it would pro-duce a vertical line where displace-ment represented voltage.

Typical of many oscilloscopes, is the facility to provide two traces. The 10-4510 fromHeathkit is a good example.

Y -amplifiers are designed to giveso many "volts per centimetre"deflection. The deflection of thebeam depends upon the voltageapplied to the deflection plates,therefore a voltage applied to theY set of plates merely shifts thespot up or down.

If a voltage is varying in timethe Y trace shows the instan-taneous voltage, whilst the X traceplots the time. Now examine Fig.3 which shows a block diagram ofthe complete oscilloscope with allthe facilities mentioned.

The oscilloscope is really adevice for plotting signals, similar

Y INPUT

TRIGGER

INPUT

AMPLIFIEROR

ATTENUATOR

SYNC. ORTRIGGER

to a pen recorder. In a penrecorder, paper (the time base) iscontinuously fed out of themachine whilst the pen shows (bydeflection on the paper) thevoltage applied. The oscilloscopeuses the time base generatorinstead of a roll of paper.

It may be useful to examine howone would plot a signal on graphpaper and then proceed to plottingthe same signal on an oscilloscope.Consider a signal present in mostelectronic equipment, a 50Hz sinewave of 10 volts peak amplitude.This is about 7 volt r.m.s.

A 50Hz sinewave means 50 com-

t=0

/1//SAW TOOTHWAVEFORM

TIME BASEGENERATOR

WITHDIFFERENT

RANGES

.J--I I-

X

POSITION

Y POSITION

FOCUS

BRILLIANCE

Fig. 3. Block diagram of an oscilloscope.

BLANKING


plete cycles or oscillations per VOLTSsecond, therefore one cycle takes 10 -1/50 second or 1000/50 milli-seconds= 20 milliseconds. This is

-a complete cycle from 0 to 360 6-degrees. + 4 -

The main signal is varying in 2-voltage according to the sine of the 0 20

1

60 100 140 180

angle therefore the instantaneous 0 40° 80° 120° ($160° 200° 240° 280° 320° 360°voltage is given by 2-

Vp sin 6VD= 10 voltsFor 6=0, sin 0=0 .'. V= 0 volts 6 -=30, sin 30=0.5 :. V=5 volts a -=45, sin 45=0.707 :. V=7.07

10 -volts

90, sin 90=1 .'. V=10 voltsfor angles between 90 and 180 useV=Vp sin(180 -6), and when thesignal goes negative we can startagain with V= lipsinei where6,=6-180 degrees.

These results are plotted on Fig.4 and readers are invited to lookup a table of sine to plot the com-plete waveform.

All the different angles 6 occurduring a fixed period of timealready worked out to 20 milli-seconds, therefore each 40 degreeinterval is given by 20 X 40 milli -

360seconds = 20/9 = 2.2 milliseconds.

If instead of degrees we plottedthe X-axis in 10cm divisions, eachdivision would represent 2 milli-seconds or 36 degrees, which al-though inconvenient to plot on agraph is much easier to plot on anoscilloscope. The oscilloscope is nomore than an instrument to plotthe results just obtained onto theface of a CRT instead of paper.

The waveform is plotted byadjusting the oscilloscope asfollows:1. Connect the sync/trigger to the

10 volt signal and adjust until a

Fig. 4. Plotting a sinewave on graph paper.

trace appears.2. Set the time base to 2mS per

cm.3. Set the Y amplifier to 2.5 volts

per cm.4. Connect the 10 volt peak sine -

wave to the Y -amplifier.5. Adjust the sync/trigger controls

to obtain a stationary trace.6. Adjust the brilliance and focus

controls to obtain a fine cleartrace.

The sinewave will now appear onthe CRT face for as long as thesignals are present.

With unknown signals, an un-known frequency for example, theprocedure is more or less the sameexcept that the time base and Y -amplifier controls are adjusted togive the best display possible andthe signal is measured against theoscilloscope calibration.

For example, suppose the timebase selected was 100mS and thenearest full sinewave measured35mS (3.5cm) its frequency wouldbe 1000/35 or 28.57Hz. In asimilar manner its voltage can bedetermined from the Y -deflection

The Heathkit 10-4530 single trace oscilloscope.

and the setting of the Y -amplifiercontrols.

In practice one seldom looks atsinewaves, signals are usuallypulses, square waves, saw toothand similar waveforms. In eachcase t=0 has to be determined tostart the time base.

With practice one becomesskilled at adjusting the instrumentand interpreting the signals dis-played. It is not possible in thespace of a short article to list allthe different signals that one canexamine with the oscilloscope,except to say in general that anysignal can be examined. Unfortu-nately, any 'scope cannot examineall possible signals as eachdifferent model is designed for arange of uses, some extremelyspecialised.

A glance at advertisements foroscilloscopes will have already in-formed readers that a wide rangeof prices are quoted. A higherprice in general will mean that the'scope can handle a wider range ofsignals, has more facilities and ismore accurate. II

The 0S4000 storage oscilloscope from Gould Advance Ltd.


SQUARE

ONELet us lead you into the exciting

world of electronics. There are thingsyou can make for yourself that willamaze you and your friends!

It all looks so mysterious. Yet thisis largely deceptive. The building partof electronics is really quite simple.True you need a certain knack andability to handle small parts and tomake soldered connections. But theseskills are soon acquired with a littlepractice.

How does one start building? Firstlook at one of the constructionalarticles in this issue. Read the INTRO-DUCTION, then the "HOW ITWORKS" section and move on to theCONSTRUCTIONAL instructions.Study these in conjunction with thecomponent layout. Refer to the COM-PONENTS list-this tells you exactlywhat to buy.

All electronic designs are based on

Here is a very simple explana-tion of this small circuit. The elec-tronic working part is a transistor,labelled TR1. This component isillustrated in Fig. 4. Note the Emit-ter, Base, and Collector lead -outwires. These are identified in Fig.1, by e, b and c respectively. Thistransistor operates as an amplifier,magnifying the electrical signal(input) applied at A sufficiently to"drive" the speech coil of the loud-speaker LS1. Power for the tran-sistor is provided from a 12V bat-tery: the positive side ("H.T. Line")is applied via the loudspeaker coilto the Collector of TR1; the nega-tive side is applied directly to theEmitter of TR1.

If you were brought up on valves,think of the Collector as the anode,and the Emitter as the cathode.The third connection to the tran-sistor is called the Base; this isvery similar in function to the gridof a valve.

The input signal applied to theBase controls the current passingthrough the transistor, and hence,in our case, the current passingthrough the loudspeaker.

... FOR BEGINNERS

WE ARE HERE TO HELP YOU -NO MATTER HOW NON -TECHNICALYOU MAY BE, JUST READ ON !

Fig. 1. The theoretical circuit diagram.This uses standard symbols andabbreviations to represent componentsand interconnection details.

0 0 0 0 0 0 0 0 0 0 000400

BREAK 011116;

00000000000ooFig. 2. Stripboard consists of paralleltracks of copper (excellent electricalconductor) on a perforated plasticsboard. The tracks are used to inter-connect the components according tothe circuit diagram. Connection of thecomponent lead to the track is by asoldered joint. Black dots indicatecomponent lead/track soldering points.

TO LOUDSPEAKERL S1

0 0 0 0 0 0 0 0 0

TR1

LINKWIRE

0 0 0 0 0 0 0 0 0 0 00

0

Fig. 3. Components are usually mount-ed on the non -copper clad side top-side) and component leads fed throughthe holes to the track beneath. Thisdiagram is called the "layout".

BFY 51

e c 0

e=EMIT TERb=BASEc=COLLECTOR

Fig.4. It is essential that the transistoris correctly connected according to thelayout (and circuit). Above shows twoviews of a BFY51 transistor to aid leadidentification. On the right is a viewlooking directly at the leads.

Fig. 5. Photograph of the underside ofa piece of stripboard containing thecircuit of Fig. 1 built up according toFigs. 2 and 3. Compare soldered pointsand break with those shown in Fig. 2.

Fig. 6. Photograph of the topside of thestripboard shown above. Importantpoints to note: reference tag on tran-sistor and positive end of C4.


a Circuit Diagram, sometimes re-ferred to as a theoretical diagram.Usually the circuit diagram isarranged so that we proceed fromcause to effect (from input to output)by reading across from left to right.

At this stage, it is not essential tounderstand how the circuit works indetail.

Our business is to translate the cir-cuit diagram into a practical formusing standard electronic components.(See components list.) The founda-tion for our building is usually a pieceof plastics board. In the example illus-trated we are using a small piece ofcircuit board known as stripboard.

Look at Fig. I. This is a smallsection taken from the FlasherBleeper, see page 24. Now look at

Fig. 3. Here we see this section ofthe circuit translated into a physicalform.

Note that Figs. 1, 2 and 3 have in-put and supply lines annotated A, Band C to aid the constructor in trans-lating from theoretical to physical.

Now to the practical work of build-ing. First we prepare the undersideof the stripboard by making thenecessary "break" in the copper stripas shown in Fig. 2. A twist drill issuitable for this.

Next turn the board and insert thecomponent leads through holes in theboard in accordance with Fig. 3. Usea pair of long -nosed pliers to bend thewires to suit. As each component isfitted, turn the board and then solder

the wires to the copper strip. Cut offthe surplus wire. That's all there is toit!

Join us here nest month for morebasic facts and helpful advice aboutelectronic circuit construction.

SQUARESfreactiPlefor BEGINNERS

ONE

Readers' Bright Ideas; any idea that is publishedwill be awarded payment according to its merit. Theideas have not been proved by us.

HEATSINKSI have thought of a way of making a heatsink

for transistors. You simply take a piece of aluminiumcooking foil, about 70mm square and wrap it tightlyaround the transistor. I find this provides an excel-lent heatsink, especially when a proper heatsink isnot available.

Ensure that the foil does not touch any of the leadson the transistor, otherwise a short will occur andpossibly destroy the transistor.

J. Kilmister,Hartcliffe,

BristolDIODE TESTER

Having constructed the Transistor Lead Out Indica-tor as described in the June issue of EVERYDAY ELEC-TRONICS, it occurred to me that it could also be usedas a diode tester. To achieve this facility twominiature screw terminals are used instead of thejack socket, these being marked anode and cathode.It is also necessary to insert a good transistor in thesocket.

In use a good diode will only give a good tone whencorrectly connected, a diode which is open circuitgives no response, a shorted diode gives equalresponse in both directions. I have found this modi-fication most useful for sorting out diodes from themany bargain packs which are sold by firms adver-tising in EVERYDAY ELECTRONICS.

L. K. Noyce,London

PCB AIDSI have recently been using a method of making

printed circuits straight from the circuit diagram.Take a piece of 0.1 inch plain matrix board large

enough to accommodate all the components. Coverone side with white Fablon. On the uncovered side,position the components approximately as they arein the circuit diagram, pushing the leads through theFablon. The points where the wires come throughthe board can now be seen, and the "islands" drawnin with a pencil, carefully connecting the properwires together.

Now remove the components and gently peel offthe Fablon and stick it down on the copper side ofthe board. Cut round the islands and remove the ex-cess. The board can then be etched in the normalway, the Fablon acting as an etch resist.

P. A. Boocock,Thirsk,

North Yorkshire

When making printed circuit boards I mark on theboard the edges of the required copper areas androughly cover them in nail varnish. When dry it isa simple matter to scrape away the excess with asharp knife. This, I find is much easier than carefullymarking the areas and applying the resist.

L. U. Barker,Solihull

CASESOne problem which is often encountered is how to

attach a finished project to the inside of it's box. Onevery easy way is to attach a piece of expanded poly-styrene foam to the inside of the box with glue. ThePCB is then placed an top of it and pressed down.The leads of some of the components should be leftlong to ensure that it remains fixed.

This method ensures to some extent that the boardis protected against knocks and bangs, and is alsoelectrically isolated from the case.

G. S. M. Potter,Wimborne,

Dorset


FREE! hit gext Wtegt1469awePOCKET DATA CARD

Pa oicNetIrleeo SneedTEACH -IN

Have you ever asked yourself "what is a transistor?. . . how does an amplifier work? . . ." If so, joinus next month for the start of a series that willexplain the fundamentals of electonics ... by theoryand experiment.

AlsoENLARGER

CIRCUIT

SYMBOLS &

COMPONENT

DATA

TREASURELOCATOR 'il(11Get on the right trail with thisinexpensive easy -to -build unit.Fun for all the family!

TIMER MODELTRAIN CONTROLLER TRANSISTOR TESTER

dgettanicsOUR OCTOBER ISSUE

WILL BE ON SALEFRIDAY, SEPTEMBER 16

Everyday Electronics, September 197721

loodidlions50 CMOS IC ProjectsAuthor R, A. PenfoldPrice 95pSize 180 108mm 102 pages paperba:kPublisher Bernards (Publishers) LtdISBN 0 900162 64 3

ALTHOUGH not entirely new to the amateur market,the wide range of cmos i.c.s is fast gaining popu-

larity amongst constructors. Due to the fact thatcmos i.c.s can operate between supply voltages of3 to 15V, and for practical purposes at zero currentdrain, they are being used in many varied projects.

Divided into four sections; multivibrator projects,amplifiers and oscillators, Schmitt triggers, andspecial devices, each section contains a multitudeof different projects ranging from a metronome,enlarger timer, audio squelch unit and many more.

As with many other books written in a similarstyle, no detailed information has been given asregards actual construction, this should not howeverprove to be a disadvantage as the circuits are allfairly easy to build.

It is unfortunate that a few mistakes have beenlet through this otherwise commendable book. It isadvisable to check the pin leadouts of the 4016 and4017 i.c.s before embarking on any projects usingthese types. T.J.J.

13atitKtuditem

We are pleased to announce that the BackNumbers Service has now been reinstated andwill operate from the issue dated June 1977.

This and subsequent issues of EVERYDAYELECTRONICS will be available at the inclusiveprice of 60p per copy. This includes inland/overseas postage and packing.

Orders should be addressed to: Post SalesDepartment, IPC Magazines Ltd., LavingtonHouse, 25 Lavington Street, London SE1 OPF.Cheques and postal orders should be madepayable to IPC Magazines Limited.

A limited supply of earlier back issues is alsoavailable. Requests with appropriate remittance,should be sent to the above address. In theevent of non -availability, remittances will berefunded.

22

SOUND11E31A PRACTICAL ELECTRONICS PUBLICATION

A SPECIAL SELECTION OFMUSICAL PROJECTS FROM PE

THE MINISONIC MK2SOUND SYNTHESISER

An up -dated version of the published Mk 1,the Mk 2 has an integral keyboard, two

250 mW monitoring channels andloudspeakers, and facilities for amplitude,

frequency and harmonic modulation.

THE JOANNAELECTRONIC PIANO

has realistic piano effect with touch -sensitivekeyboard and additional choice of harpsichord

or honky-tonk voicing.

THE ORION STEREO AMPLIFIERA hi-fi amplifier with output of over

20+20 watts. Compact and complete in oneunit, it measures only 14" x 6" x 2".

PLUSSome great sound effects units for guitars,

keyboard instruments and general recording.

r

AvailableNow

£1.20POST PAIL)

(Please allow at least 2 weeksfor delivery)If you do not wish to mutilate yollfcopy of the magazine, please sendyour order on a separate sheet.

To : Practical ElectronicsIPC Magazines Ltd., Receiving Cashiers Dept.,

King's Reach Tower, Stamford Street, London SE1 9LSPlease send me copy(ies) of

"Sound Design". I enclose a Postal Order/Chequefor £1.20 (post paid) or (state amount for

more than one copy) (£2.35 post paidfor 2 copies)

PLEASE WRITE IN BLOCK LETTERS

Name

Address

1

Post codeRemittances with overseas orders must be sufficient to cover despatch by

sea or air mail as required. Payable by International Money Order only.EE.1510

Company registered in England. Regd. No. 53626.A subsidiary of Reed International Limited.


By DERRICK DAINES

F ALL the gadgets made by youngstersentering the hobby of electronics,

the shocking coil is certainly among thetop three favourites, so no excuse isneeded to introduce the item here. Thereare several ways of making such a deviceand one that is probably the easiest tounderstand is outlined in Fig. 1.

Shocking coilsAll shocking coils for amusement have

in common the provision of a very highvoltage at an extremely low current and assuch are perfectly harmless. It must bestressed however that multiplying thesupply voltage by, say, a factor of ten alsoincreases the total power even if thecurrent is still very low.

SELL MOVEMENT

TRANS FORMER

Moreover, it appears that some peopleare less tolerant towards the effect ofshocking coils than others. It is for thesereasons that the supply voltage must notbe increased above that recommended-certainly in no case should it exceed 4.5volts.

It would be a great pity if at the outset ofan interest in electronics we cause the ireof the household authorities with perhapsthe subsequent ban on all future experi-ments!

We therefore utilise small torch bat-teries for the supply, 1, 2, or 3 will givevoltages of 1.5, 3.0 and 4.5 volts respec-tively. However, a bell transformer will notwork with d.c., so some means must befound to produce a.c. or trains of pulseswhich will be stepped up by the trans-former.

Fig. 1. Utilising a bell transformer a simple (and effective!)shocking coil can be made.


HAND-HELDMETAL PROBES

One way is to pass the current througha small d.c. motor, thereby chopping -upthe d.c. by the spinning of the armature.Another way is to feed a bell movement,as shown in Fig. 1. Notice that the belltransformer is operated in the reversemode i.e. as a step-up transformer. Tothis end the chopped -up battery supply isfed into the winding that normally is con-nected to the bell, while the hand heldprobes are connected to the windingnormally connected to the mains supply.

In use two, three or four people holdhands with the person at each end grasp-ing a probe. All will then feel the high -voltage tingle. If any body protests that itis decidely unpleasant, reduce the supplyvoltage by removing one battery cell.

Self InductionSooner or later it will occur to the young

hobbyist that he is using two coils, one tochop -up his d.c. and the other to transformit to a higher voltage. Is it possible, he willwonder, to make one coil do both jobs?Indeed it is and last month I promised toshow how to make a shocking coil usingthe self-induction principle. The circuit isshown in Fig. 2.

All that is needed is a bell movement,which can be either taken from a discardedbell or made up in the manner shownearlier in this series. Since the heart of themovement is a very efficient electromagnet, it is also good for producing thephenomenon of self-induction. In Fig. 2. Ihave not shown the connection to one ofthe hand-held probes. This is because itwill work connected to either point A orpoint B of the circuit.

Connected to one of the points theprobe will gather the self-induced currentgenerated when the magnetic flux buildsup; connected to the other point it willgather that produced when the magneticfield collapses. Can you determine whichis which? If you cannot, try each in turn.Remember that the self-inductance gen-erates more current when the field col-lapses than when it builds up.

Self-inductance is sometimes referredto as back e.m.f (electromotive force) andwhen sensitive electronic devices are in acircuit containing a coil it is very fre-quently necessary to insert a diode ex-pressly to discharge the back e.m.f.Otherwise, the device would be destroyed.

But there-you will be able to feel thepower of the back e.m.f for yourself!

OB

Fig. 2. A similar circuit to Fig. 1 but in this case uses theprinciple of self-induction.

23

I- By T. R. de VApX-BAILBIRNIE B.Sc.

INTRODUCTIONWHEN the author drives the

family motor caravan, it hasbeen known for him to drive formile after mile with the flashingindicators operating. The reason isthat they are not self -cancellingand the "tick" from the flasherunit is rather quiet especiallywhen driving at speed. The author'swife has unkindly intimated thathe is going a little deaf-asuggestion which the authorvehemently denies.

Whatever the reason, it seemeda good idea to devise an electroniccircuit which emits a loud warningnote in time with the flashing indi-cators. This project could be foundvery useful to those in a similarsituation, those a little hard ofhearing or those who drive old andnoisy vehicles. It could be of realbenefit to van drivers. The indica-

tor light on the dashboard is oftenbadly positioned so that it is ob-scured by the steering wheel or istoo dim to attract attention inbright light. Similarly, the indica-tor unit itself may be badly sitedso that the "ticks" are too quiet.

When designing the circuit, itwas thought necessary to providetwo special features. One is avolume control so that the nor-mally loud warning signal can beadjusted to the particular applica-tion. It was also thought necessaryto fit an on/off switch so that theunit can be completely silenced asrequired. Sleeping babies orelderly passengers may take ex-ception to electronic bleeps. Itmust also be remembered thatothers may drive the vehicle andmay not require the device. A pre-set control is provided on the cir-cuit panel by which the pitch ofthe warning note may be alteredto suit individual taste.

GoDANc

L,0 ESTIMATED COSTOF COMPONENTS

23

The design of the original cir-cuit was made with discrete com-ponents but found that it sufferedfrom changing pitch as the volt-age varied. This meant that itchanged its note abruptly as theengine was revved up or the brakelights were operated. The circuitwas effective but this defect wasvery annoying. To incorporate astabilizing circuit seemed hardlyworthwhile. It was finally decidedto use an integrated circuit as atone generator. The type chosenwas the low-cost 555 Timer, an 8pin d.i.l. device normally used forshort period timing circuits. Itmay, however, be wired as aneffective astable multivibrator. Thecircuit is very tolerant to voltagechanges which makes it ideal forthis type of application. It hardlychanges its tone with quite widevariations in supply voltage.

STARTHERE FOR

CONSTRUCTION

The suggested layout is based on0.1 inch pitch stripboard, see Fig.1. It is essential to cut the copperstrips in the places indicated usinga proper tool or with a small twistdrill turned by hand. The i.c. maybe soldered direct to the circuitpanel but the use of an 8 pinsocket is advised.

Whether using an i.c. socket or


not, care should be taken not tocause bridging between the pinsor any of the copper strips in thecircuit panel. The two way connec-tor shown in the illustration isadvised as it allows for easy instal-lation and removal in the event offailure.

After constructing the project itwill be necessary to find a suitableconnection to power it. This iseasy if the car is fitted with asingle warning light on the dash-board which flashes for both rightand left turns. Most cars thesedays have this sort of arrangementand the prototype is used on one.

The "live" connection will thenbe taken to the non earth side ofthe bulb or from the correspond-ing terminal on the flasher unit.

A

B

C

E

F

H

I

J

K

1 2 3 4 5 6 7 8 9 10

0000000 00000 VR1 R1

ResistorsR1 22k12.

R2 1.5kS1R3 5.642

COMPONENTS''

All resistors are 4-W carbon ± 10%

PotentiometersVR1 100K2 horizontal miniature presetVR2/S1 10k0 carbon log. with ganged single pole switch

CapacitorsC1, C2 0.01/2F polyester (2 off)C3 0.1/.4F polyesterC4 470,uF 15V elect.

SemiconductorsIC1 NE555 timer i.c.TR1 BFY51 silicon npn

MiscellaneousLS1 miniature loudspeaker 40 to 70 ohm coil impedanceStripboard 0.1 inch matrix 12 strips x 35 holes; 8 pin i.c. socket; small2 way 5 amp block connector; case to suit; stranded wire; solder.

Page 13

0 0 0 0 0 0 0 0 0 0 0 0 0 ".

Fig. 1. The layout of the components on the stripboard and thebreaks to be made on the underside of the board includingwiring up details to other components and the flasher unit.

K

J

I

H

F

C

B

A

O000000000

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 00 00 00 0O oo 0 oO OO 0 OO 0 Cl 0 Cl 00 00 CI o0000000000000000000013o 000000

TO FLASHERUNIT-POSITIVE

EARTH

UNDERSIDE VIEWOF TR1


FCIRCUIT

DESCRIPTIONThe output from the i.c. is fed

via a capacitor to the volume con-trol and from there to the base ofthe output transistor. Resistor R2deserves mention. Without it, thevolume control needs considerablerotation before any signal is heardat all. This means that the adjust-ment from very quiet to fullvolume happens over quite a smallpart of the track. It would be quitepossible to fit a preset variable ofvalue 10 kilohms instead of thefixed resistor R2 so that the exactoperating characteristics would beadjustable.

The fixed resistor of 1.5 kilohms,however, works very well in theprototype. With the control justturned from the off position thesignal is very quiet and adjusts tofull volume over the entire rota-tion.,The transistor chosen for the

prototype is a rather "hefty" oneand the experimenter may wish touse a different type than thatspecified. It is likely that a ZTX300would serve as an alternative. Theloudspeaker is a miniature 50mmtype with an impedance of 70ohms or so. It was found possibleto build the whole unit into a small

HOW IT WORKS...

VR1700kI2

22kr:

CI mom0,91).JF

ICINE55SV

5 j -m.-

3

2 C3

sum C2

VR2iookn

R21,5k1)

R35-6kI2

LS140 TO 11011

TR1BFY51

CA47OpF

TO FLASHERUNIT

plastic box with small holes farthe sound. The method of securingthe volume control will depend anthe type used.

It will be noted that the unit isequally at home in a positive ornegative earth vehicle. The posi-tion of the on/off switch as givenin the circuit diagram is for nega-tive earth vehicles. It would befitted in the negative lead if theunit were fitted to a positive earthcar.

The purpose of capacitor C4 isto suppress clicks. Without it, theloudspeaker is likely to emitannoying. clicks each time theflashing indicator unit operates.

BLEEP....BLEER.

When the car indicator arm is operated to indicate an intended changein car direction, the indicator lamps are made to flash on and oft by ap-plying, via the flasher unit, a pulsed 12 volts to the indicator lamps. Nowthe Flasher Bleeper consists of an oscillator whose power supply line isconnected to a pulsed 12V point in the indicator system. Therefore eachtime the lamps flash on, the oscillator is turned on and produces anaudible tone in the loudspeaker. The output from the oscillator is fed toan amplifier fitted with volume control to allow for varying ambient noiseconditions.

This is largely eliminated by in-cluding the capacitor, although itadds to the cost, it is worthwhile.The suggested value is 470 micro -farads. For those with junk boxes,there is scope for trying differentvalues but that given should be-have well. If its value is too small,the clicks will not be properly sup-pressed. If it is too large in valuethe bleeps from the loudspeakerwill not start and end sharply asthey should- but will "grow" and"decay" rather slowly giving anunpleasant effect.

The polarity of the electrolyticcapacitor must, as always be ob-served.

If the car has two indicatorlights, one which flashes for theleft indicators and one for theright, the constructor must investi-gate at the flasher unit to findthe connection which gives thedesired effect. This may be donewith a small 12 volt test bulb oneconnection of which is earthed toa suitable point.

Both the live and earth wiresmust be of the stranded type,single wires soon fail under vibra-tion. Note that the indicator lightitself forms an important part ofthe circuit in providing a dischargepath for the electrolytic capacitor.If the constructor tests projectslike these with dry batteries on thetable before installing them in thecar it will be necessary to fit asmall 12 volt bulb across the posi-tive and negative connections toobtain the true effect. Withoutthis, the curious "growth" and"decay" effect referred to in theCircuit Description will occur.


Photograph of the prototype Flasher Bleeper.

Modern flasher units tend to bevery current sensitive. This doesnot matter at all if the unit ispowered from the indicator lightcircuit on a three terminal flasherunit. On a two terminal flasherunit, however, the current con-sumed by the unit adds itself tothe total drawn by the main lights.The current drawn by this projectis so small that there will be noupset here-it is only about 100mA depending on the setting ofthe volume control.

Flashing indicators left goingare a danger to other road users.This project will make sure thatyou do not offend in this way! ):z

Estimated CostMy son, aged 13, is very keen on elec-

tronics and we have a standing order foryour magazine every month.

I know the prices you give are approxi-mate, but I would like to point out that theFish Attractor in the June issue, you givethe approximate price cost as £3.00 but upto now it has cost him £7.50 and he stillhas three components to buy.

I thought you might be interested toknow this as he bought the componentsfrom Tandy's.

Mrs. B.H. Shilling,Aspley,

Nottingham.

We sympathize with you and your son forspending such a great deal of money.

All the prices for the constructional pro-jects are taken from advertisers in EVERY-DAY ELECTRONICS, not just one but many.In this way the price of each project is kept toa minimum. Certainly our prices in the mainare cheap, but in many circumstances con-structors have even made the projects forlessl The moral therefore is to shop around.

T.V. OscilloscopeI recently discovered that by using an

old u.h.f. television it is possible to con-struct a fairly sensitive oscilloscope. Themethod I used was to plug the frame out-put generator into the line deflector coils.

The frame coils being connected toprobes. The sensitivity I obtained was136m Vicm.

A. Hill,Chelmsley Wood,

Birmingham.

The idea of using a redundant tv as anoscilloscope is of course well known, adesign for one was published recently in oneof our companion magazines.

It should be realised that due to the in-ternal circuitry of the tv, the available band-width is severely limited. The best you couldhope for is in the audio frequency range. Aword of warning, the innards of a tv are po-tentialy dangerous, so unless the constructorknows what he is doing, this modificationshould not be tackled by the beginner.

Can You Help?I recently bought the July issue of

EVERYDAY ELECTRONICS, and I am particularlyinterested in the PhonelDoor BellRepeater

Being deaf I am interested to know if thecircuit can be modified to operate a lampwhen the phone rings.

W. Westgate,57, Kelsull Croft,

Chelmsley Wood,Birmingham.

It is not our standard practice to providereaders with modifications to our publisheddesigns. However we feel sure that amongour many readers there is someone whocould come up with a design suited to yourneeds. If any reader does so they are asked towrite to the above address.

Cmos i.c.'sIn your article, Electronic Dice, published

in the March issue of EVERYDAY ELEC-TRONICS the i.c. type CD4017 has threeunused tags.

As the device is static -sensitive, shouldthese tags be connected to earth? Hopingyou can advise.

A. Goudge,St. Albans, Herts.

You are quite right in saying i.c.s of thecmos type are sensitive to static. In thiscase however the tags referred to are outputs,and therefore need no protection.

It is however a different story when inputsare left open. In very severe cases leaving theinputs open can cause the entire i.c. tooscillate, and in some cases the i.c. mayoverheat through excessive current drainand destroy itself.

The simple remedy is to connect allunused inputs either to the negative orpositive supply line, whichever is appropriatefor the logic circuit involved.

Amateur Radio ClassesAn amateur radio and theory and morse

class is to be run by the Newcastle uponTyne Education Committee. The classeswill be held at the Gosforth Adult Asso-ciation Classes at the Gosforth HighSchool, commencing in September 1977.

Although specifically for the RadioAmateurs Examination, the course isideal for anyone wanting to get an insightinto radio theory, having just taken upradio or electronics generally as a hobby.

The theory class will be held on Tues-days of each week from 7pm to 9pm, themorse class will be held on the Thursdaysof each week at the same times.

Enquiries should be addressed to; ThePrincipal, Gosforth Adult Association,Gosforth High School, Knightsbridge,Gosforth who will arrange for furtherdetails to be sent on application. Alter-natively further information can be hadfrom myself by telephoning Newcastleupon Tyne 668439.

D. R. Loveday(G3FPE)

Remember that "ReadersLetters" is your page foryour news, views andcomments. So don't beshy, write to us, we will bepleased to hear from you.


ANY traveller waiting on the rail-way platform at Lord's Bridge

Station, Cambridgeshire, will bedisappointed for no trains will evercall there again, and if that soundslike the end of the story, it isn't.In fact it's the beginning of one.

For the site is now the home ofthe Mullard Radio Astronomy Ob-servatory (MRAO). Founded withthe aid of a generous benefactionfrom Mullard Ltd, but adminis-tered by the Science ResearchCouncil, it is indeed the end of acommunication link that stretchesback much farther than Cambridgeor Bedford, but quite literally tothe stars.

It shares a unique place in theforefront of international radio -astronomy with the only othermajor British installation, JodrellBank in Cheshire.

The two observatories do indeedinvestigate a common science butthey have differed considerably indevelopment. There are no massivesteel dish aerials at Cambridge likethe very famous Mark IA JodrellBank reflector 250 feet in diameter.

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ificz., imm ,

Instead we see several groupsof much smaller aerials at Lord'sBridge. Just how does this arrange-ment offer a valuable alternativeto the big dish approach?

AERIAL SYNTHESISWe can well understand the

radio astronomer's yearn for biggerand bigger telescopes. With bigdishes, he can pick up faint radiosources far away in the universe-more importantly he can resolvethe fine detail of the sources healready knows about. So on bothcounts, the bigger the instruments,the better.

However, radio waves have avery much longer wavelength thanlight waves. This means that ahigh resolution optical telescopeneeds a mirror of only a few feetacross but a comparable radio tele-scope would need a reflector dishseveral miles in diameter. Buildingsuch a dish would be an engineer-ing nightmare and even if it werepossible the cost would, quite

literally, be astronomical.However there is an ingenious

way out. In 1946 a research teamled by the radio astronomer SirMartin Ryle realised that such adish could be broken down ineffect into two or more muchsmaller dishes spaced out. Onlyone line of these dishes is actuallyneeded because the earth's rota-tion will swing it round in a circlewith respect to a radio source. Theresult is shown in Fig. 1.

We reform the big dish in out-line and can "fill" it in if we com-bine observations made at differentspacings of the small dishes.

This is the basic principle ofaperture or aerial synthesis.

LOOKING AROUNDAt Lord's Bridge most of the

telescopes work on the aboveprinciple. Thus the so-called "OneMile Telescope" actually consistsof three separate dishes or reflec-tors: one moveable one on a rail-way line and two fixed. This

x RADIONOW

By S. McCLELLAND


ROTATION OF THEEARTH AS SEENFROM RADIO SOURCE

SOUTHPOLE

DISHt.,.),) FROMBIG APPARENT

MOTION OF SMALLDISHES

Fig. 1. The principle of Aerial Synthesis. To a radio source far out in space, two smallaerials spaced out are combined by the Earth's rotation effectively into one large one.

arrangement can give all dishspacings from zero to half mileand from half to one mile, i.e.this arrangement is, equivalent toa big reflector of 1 mile diameter.

The "Half Mile Telescope" asso-ciated with it has a group of 4 re-flectors, two fixed and two move-able ones and works similarly.However, the technique hasreached its highest level of sophis-tication in the most recent of theCambridge telescopes-the 5kminstrument, completed in 1972.This has 8 dishes-four fixed andfour moveable-spread out over adistance of just over 3 miles. Itwill be described in more detaillater.

The point is with each of theabove arrays, individual reflectorsizes are small, 60 feet, 30 feet,and 42 feet for the One Mile, HalfMile and 5km telescopes respec-tively. Yet the 5km telescope isquite capable of achieving the re-solving power of the world's mostadvanced optical telescopes.

OUT IN SPACEHowever, after all the effort of

building a telescope it would notbe unreasonable to ask "What'sout there to study?"

The simple answer could almostbe "More than we could ever findout".

For the stars that the casualobserver sees in the night sky arethe few and relatively insignificantrepresentatives of the 10,000 mil-lion other stars in our galaxyalone. The truth is that there arethousands of other galaxies. Their

distances are so great that theyare unimaginable as the quantiesof energy they must pour out forus to detect them.

A special category of thesepowerhouses are the "radiogalaxies" which, as their namesuggests, are very strong radioemitters. The prima donnasamongst even these are themysterious quasars-star like ob-jects, which can be over a milliontimes brighter than ordinary stars.

Above all, we realise that theuniverse is not static but is con-stantly expanding. Everything ismoving away from everything else

and the farther away it is, thefaster it moves. Indeed some ofthe very distant quasars appear tobe receding at velocities very nearthat of light itself.

Enormous distances, huge num-bers and intense energies, all areset pieces in the cosmic battle-ground over our heads. The radiowaves which tell us so much abouttheir origins tell us one thingmore-about the past. For evenlight and radio waves travelling at186,000 miles per second can takean appreciable time to cross thevastness of space. The result is wesee the universe not as it is butas it was in the past and the far-ther we look out into space themore we look back in time.

We see our own sun as it was8 minutes ago, but the light fromits nearest neighbour star took 4years to reach us. And from theAndromeda nebula -a nearbygalaxy-the picture the Cambridgeradiotelescopes now receive beganits journey two million years ago!

Indeed the most distant objectsof all at the very edge of theobservable universe can tell uswhat the universe was likethousands of millions of years agoand perhaps even how it began.

It is for this reason that deepspace surveys have been made bythe Cambridge radio astronomersand their 3C, 4C and 5C radiosource catalogues are used inter-nationally as a source of reference.

Professor Anthony Hewish, Nobel prizewinner, at the controls of the One -MileTelescope. B/CC photograph


on rails.

It is found at great distances outin space, radio sources become farmore numerous. This seems toindicate that some 13,000 millionyears ago all the matter in theuniverse was much more concen-trated than it is now, perhaps evenin a gigantic fireball which blewapart in a so-called "Big Bang"explosion.

Fascinating enough but perhapsthe MRAO's most spectacular dis-covery came in 1967 when radioastronomers realised that somestars could tick like clocks-thepulsars.

PULSARS AND BLACKHOLES

It is ironic that the first pulsarwas discovered by a radio telescopethat had no dishes at all.

The "Four Acre Array" at Lord'sBridge instead consists of 2048 di-pole aerials at an approximateheight of 6 feet strung togetherlike so many clothes lines along-side the One Mile instrument. Itwas built to investigate small dia-meter radio sources, but super-imposed on the pen -recording out-put was a series of blips.

These blips were very regular,

their periodicity was at least asgood as the most advanced clockson earth.

After a thorough investigation,the scientists realised with excite-ment and perhaps some trepida-tion that those signals were notindeed produced on earth.

Where could they have comefrom? At first the recordings weredubbed LGM-(Little Green Men)which shows that one possibilityhad already crossed the minds ofthe scientists involved.

The truth is probably strangerstill. Astrophysicists believe thatthese signals are produced by deadstars, which have ended their livesin a huge blaze of glory or super-nova leaving behind a tiny hulk.This is made of matter so con-densed that it's been picturesquelysuggested that a teaspoonful of itwould weigh 10 million tons!

The radio blips arise becausethis hulk, a so-called neutron staris spinning and is throwing offelectromagnetic radiation incrests as it does so, like an earth-bound lighthouse beacon.

Pulsars are by no means un-common, their numbers now runinto three figures and more arebeing discovered all the time in

Five of the aerials of the 5km telescope: four of these are mountedBICC photograph

Aerial view

the various "pulsar hunts" beingconducted in various radio astro-nomical observatories around theworld.

However, as is often the case inscience, theory has outstripped ex-periment and is waiting for it tocatch up. In this case, theory pre-dicts that for some, neutron starsmay not be the final resting state.

For these, the gravitational pres-sures may be strong enough toforce them all the way into thebizarre nothingness of those ob-jects seemingly straight fromscience fiction-the black holes.

We must wait for experimentalevidence to confirm the existenceof these objects.

NOBEL PRIZEThe pacemaking work of the

Cambridge group was given inter-national recognition in 1974 whentwo of its members were awardedthe Nobel Prize for P ysics.

The award was m e jointly toProfessor Sir Marti Ryle theAstronomer Royal, who as head ofthe radio astronomy group, waslargely responsible for its initia-tion and the development of theaperture synthesis techniques;

of a radio -mounted dish of the 5km telescope.BICC photograph

and Professor Anthony Hewish forhis contribution to the discoveryof pulsars.

THE 5km TELESCOPEWe can now bring the story of

Cambridge radioastronomy up todate by discussing the most recentand advanced instrument to bebuilt there, the 5km telescope.

Like the others it is built onthe aperture synthesis design andits eight dishes (four fixed andfour moveable) are spread out atLord's Bridge along the site of theold Cambridge to Bedford railwayline.

Its designers have gone to greatlengths to ensure that it is one ofthe most accurate instrumentsavailable. This accuracy necessi-tated surveying the site with un-precedented precision, and thiswas carried out by the OrdnanceSurvey. They managed to make allmeasurements with an error ofless than 1cm over 5km.

In fact this was still insufficientbut by recalibrating the dishes(when the instrument was set up)against known radio sources in thesky the positioning error wasbrought down to 0.3 millimetresover the same distance! An objectlesson indeed in measurement. Noless was the accuracy aimed forand achieved in the operation ofthe telescope, low noise electronicequipment of high stability is inuse throughout the telescope sys-tem to take advantage of this.

The radio waves received byeach dish are focused by theCassegrainian reflector (positionedon the projecting tripod) and fedinto the central receiver.

These receivers are special solidstate amplifiers of the parametrictype. Signal amplification at theselected frequency is achieved byactually driving the amplifier attwice this frequency. So, for aworking wavelength of 6cm i.e.4995GHz, the amplifier is operatedat 9990GHz. After mixing withsignals from a local oscillator inthe control room the final outputfrequency is about 45MHz.

CONTROLThe whole network of dishes

must be kept "locked on" to theradio source it is studying and thisnecessitates the use of an on-linecontrol computer which con-tinuously adjusts the position of

The Director of the Mullard Radio -AstronomyProfessor Sir Martin Ryle.

each dish to compensate, for ex-ample, for the rotation of the earth.

The computer oversees everyaspect of the instruments opera-tion. It even keeps the system oftelescope dishes electrically "bal-anced" by automatically insertingcable sections as required, fromthe 60 miles of cable network thatruns between the control roomand the eight dishes. This is toensure that the path to each dishis constant no matter where thedishes themselves may be pointing(necessary for bandwidth reasons).

Aware too of the fact that theradio telescope is sensitive enoughto pick up any noise signalsgenerated by the computer itselfinto a room lined with copper -cladwalls.

The second function of the com-puter is to process the radio sig-nals received by the telescope intomeaningful output which can befed to a graph plotter for example,to display a "picture" of the radiosource in terms of contours linkingpoints of equal radio intensity asan ordinary contour links equalheights.

LOOKING TO THE FUTUREDue to the accuracy which the

aperture synthesis techniqueaffords it has been possible to mapthe structure of many radiosources in much finer detail thanpreviously possible. More informa-tion can often be gleaned if radio

Observatory and Nobel laureate,BICC photograph

source results are compared withoptical observations of the samearea, but intriguingly many opti-cally bright objects are weak radiosources and the converse mayequally be true.

Cygnus A, for example, is agalaxy system and one of thebrightest objects in the radio sky.It has two very strong radiosources apparently moving inopposite directions but all that canbe seen with an optical telescopeis a small, faint central galaxy.

Another strong radio source isCassiopeia A. Here the radiotele-scope shows a distinct circularshell of matter gradually spread-ing outward: it is another remnantof a supernova explosion.

Just what is going on insidethese sources? Scientists can onlyspeculate. Without more informa-tion they are uncertain as towhether even the basic laws ofphysics hold under such extremeconditions.

As always we must look to thefuture for this information, thepieces needed to complete thetantalising puzzle Nature has setout before us. I1

AcknowledgementThe author wishes to acknow-

ledge the co-operation given to himby the radioastronomy group ofthe Cavendish Laboratory, Cam-bridge, and the Mullard RadioAstronomy Observatory.


Add-onCAPACITANCE UNIT

INTRODUCTIONTHERE can be little doubt that

the single most useful piece ofelectronic test equipment is themultirange test meter. However,multimeters do have distinct limi-tations, and one of their primaryones is that very few are equippedto measure capacitance.

This unit has been designedto enable any multimeter thatpossesses a 0 to 50 microamp

HOW IT WORKS...CAPACITORUNDER TEST

SQUARE WAVEOSCILLATOR

range to measure capacitance inthe following three switchedranges; Range 1:0 to 0 05/AF;Range 2 : 0 to 0.5i1F; Range 3 : 0 to5)1F. This covers most of the valuesthat are likely to be encounteredin general electronics work, withthe exceptions of electrolytic typesand very low value components.The former can usually be testedsatisfactorily using a multimeter

5.6V

OV

MULTIMETER

The Add -On Capacitance Unit consists of a square wave oscillator with avital component missing-a capacitor. By inserting the capacitor to betested into the circuitry, the oscillator is completed and an output results.The output consists of a train of pulses and the value of the capacitor willdetermine the frequency of oscillation and mark/space ratio.

The signal from the oscillator is fed to a multimeter set at 50pA d.c. (or apanel meter of same rating). As the pulses are produced at a rate of hun-dreds per second, the meter movement responds to the average outputvoltage rather than to individual pulses and so the result is the same as ifthe signal was a d.c. one.

By R. A. PENFOLD

without any adaptor, and the latterare more rarely encountered, andtheir measurement is really beyondthe scope of a very simple instru-ment such as that described here.

A minimum of components areused in the circuit which is, in con-sequence, both simple and inexpen-sive to construct. The unit shouldprove to be a very useful additionto one's test gear.

STARTHERE FOR

CONSTRUCTION

A ready made 133 X 70 X 38mmaluminium case is used as thehousing for the prototype adaptor.This is about the smallest size thatcan be used, and it would be advis-able for the inexperienced con-structor to use a somewhat largercase. Apart from this virtually anycase can be used. The symmetri-cal panel layout used by the authorgives an attractive appearance, butthe exact layout used is not criti-cal.

All the resistors and semicon-ductors are wired up on a 0. 15inchmatrix stripboard panel having 13copper strips by 15 holes. Detailsof this, together with details of allthe other wiring are shown in Fig.1.

Start by cutting the board tosize using a hacksaw and then drillthe two 6BA mounting holes usinga 3mm twist drill. Then makethe six cuts in the copper strips. Ahand-held drill bit (about 6mm dia)


GooAnick.

44° ESTIMATED COSTOF COMPONENTS

£3

can be used to do this if the specialspot face cutter tool is not avail-able. Next the resistors and thenthe capacitors are mounted andsoldered in. Veropins are used atthe points where Si, SKI, etc., willeventually connect to the panel.

Mount the component panel atany convenient place inside thecase using short insulated spacersover the mounting bolts in orderto hold the panel a little wayclear of the bottom of the case.Make sure that the panel is posi-tioned somewhere that permitseasy access to the three presets.

Finally, the unit is wired up toconform to Fig. 1 using ordinaryinsulated connecting wire. Capaci-tors Cl to C3 are wired up on S1which is a standard 3 -pole 4 -waywafer switch.

ADJUSTMENT AND USEThe test prods of the multimeter

are connected to the unit by acouple of ordinary insulated leadswhich are terminated in a 3.5mmjack plug at one end and a coupleof crocodile clips at the other. Thejack plug is inserted into SK2 andthe crocodile clips are connectedto the test prods. Make sure thatthe test prods are connected withthe correct polarity (positive prodto the wiper tag of Slb (via SK2).

Internal photograph of the completed Add-on CapacitanceUnit, showing the layout used.

SK1

BATTERY CLIP

TEST0 TERMINALS

A

0

F

1 3 4 5 6

o 00000

cliti 0&-41TR2

P4 4

9 10 11 12 13

0 0 0 0 0

VR3

01

©HVR2

R3Olde

J 0 TR1 07-1 O.J0 0

R1l -J

\11

V

00 00 00 00000

B 12 11 10 9 8 7 6 5 4 3 2 1

0

A

0 0 0 0 0 0 0eN...

0 Or0 00 0

0 00 00 s 0

0 0 0 0

03

0 00 0

e...., 0

0 0 0 0_0 0_0 0 0 0 0 0_0

Fig. 1. Stripboard layout showing breaks required, and the various interconnections betweenthe components on the front panel.

411.111ft 1111111111111111%

Three photographs taken from the screen of an oscilloscope. The waveform shown is that appearing at the output, when different value ofcapacitors are connected. Note in particular the increase in length of the bottom of the waveform, this corresponds to an increase of

capacitance.

Everyday .E1o7trcmies, September 1977 33

CIRCUIT DESCRIPTIONReferring to Fig. 3 this shows

the complete circuit diagram of theadaptor, and is based on an astablemultivibrator. The operation of anastable circuit is probably mosteasily explained with the aid ofthe skeleton circuit shown in Fig. 2.

The circuit consists basically oftwo single transistor stages. Onestage uses TR1 with R1 as its collec-tor load and R2 as its base biasresistor, and the other stage usesTR2 with R3 and R4 in these re-spective positions. The output ofeach stage is coupled to the inputof the other stage via a d.c. block-ing capacitor. These are Cl and C2of Fig. 2.

Fig. 2. Basic multivibrator circuit.

When power is initially con-nected to the circuit both tran-sistors will begin to turn on, asthey will both be receiving a basecurrent via the appropriate resis-tor. Due to component tolerances,one transistor will begin to turnon faster than the other.

For the sake of this explanationwe will assume that it is TR1 thatturns on the faster. As it does soit will send a negative signal tothe base of TR2 via Cl. This hasthe effect of turning TR2 off. TR1thus turns hard on while TR2 isheld in the off state.

When the collector potential ofTR1 has fallen to virtually thenegative supply rail voltage, ob-viously no further negative signalvoltage can be applied via Cl toTR2 base. Capacitor Cl thereforecharges by way of R3 until about0.65V is present at TR2 base. TR2then begins to turn on and a nega-tive signal is supplied via C2 toTR1 base. This turns TR1 hard off,and the positive signal producedacross RI is fed by way of Cl toTR2 base, turning TR2 hard on.

Capacitor C2 then begins tocharge through R2 and when about0.65V is present at TR1 base, TR1begins to turn on. A regenerativeaction similar to that which justoccurred will now take place,

DV

except that it is now TR1 thatfinishes hard on, and TR2 thatfinishes hard off.

The similarities between thepractical circuit of Fig. 3 and theskeleton circuit of Fig. 2 shouldbe obvious. There are a few addi-tional components in the circuitof Fig. 3, such as zener diode Dl.This ensures that the peak outputvoltage at TR2 collector remainsreasonably constant, and is notgreatly affected by variations inthe supply voltage.

If we consider the unit switchedto Range 1 (Si in position 2 andcapacitor Cl in circuit), with a0 capacitor in circuit, VR1 isadjusted to produce full scale de-flection of the meter. If a lowervalue capacitor is now connectedacross the test terminals, say0.01,Z, the output pulses (and thusalso the av

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Transcript of Easy to build projects for everyone - WorldRadioHistory.Com · 74 SERIES TTL IC's TESTED TRANSISTOR...